<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        ul {
            list-style: none;
        }
        ul li {
            height: 40px;
            width: 150px;
            background-color: aqua;
        }
        .menu-trigger {
            position: relative;
            cursor: pointer;
        }
        .menu-trigger>ul li {
            background-color: blue;
        }
        .dropdown-menu {
            position: absolute;
            top: 100%;
            left: 0;
            /* 透明有交互不脱离文档流 */
            transform: translateX(-160px);
        }
        .dropdown-menu li {
            height: 40px;
            background-color: aqua;
            margin-bottom: 2px; 
            opacity: 0; /* 初始透明 */
            transform: translateX(-160px);
            /* 过度属性，持续时间，过度曲线，延迟时间 */
            transition: all 0.2s linear calc(0.1s * var(--i));
        }
        .menu-trigger:hover .dropdown-menu li {
            opacity: 1;
            transform: translateX(160px);
        }
    </style>
</head>
<body>
    <div class="menu-trigger">
        <ul>
            <li></li>
        </ul>
        <div class="dropdown-menu">
            <ul>
                <li style="--i:1"></li>
                <li style="--i:2"></li>
                <li style="--i:3"></li>
                <li style="--i:4"></li>
                <li style="--i:5"></li>
            </ul>
        </div>
    </div>
    
</body>
</html>